Model for predicting phase transformation and yield strength of vanadium microalloyed carbon steels

An integrated model for predicting recrystallization, phase transformation and yield strength of vanadium- microalloyed carbon steel (V-steel) is developed. Two effects of vanadium addition on recrystallization are assumed: one is the solute-drag effect on mobility of grain boundary, the other is pinning-effect on austenite grain growth due to vanadium carbide (VC) precipitate in austenite. The austenite grain size is considered as the control variable for nucleation density in grain corner, grain boundary, and grain interior during phase transformation. Thermodynamic data for transformation including para-equilibrium of carbon concentration and driving force were calculated using ThermoCalc software. The vanadium addition leads to α/γ -interphase VC precipitation in ferrite, which accelerates the diffusion rate of carbon in austenite at α/γinterface and increases nucleation sites for intragranular ferrite transformation. In consequence, the ferrite fraction and grain size are increased. Brandt model²⁷⁾ is conducted to predict pearlite transformation. The lamellar spacing is considered as a function of carbon concentration and undercooling. The alloying elements, ferrite and pearlite fractions, ferrite grain size, and lamellar spacing were taken into account for predicting strength of V-free steels. Modified Ashby-Orowan equation is then used to calculate the VC precipitation strengthening of V-steels. Using this model the calculated results obtained are in good agreement with experimental results.